Down the hole drill



June 9, 1964 Filed June 7, 1961 E. B. LEAR ET AL 3,136,375

DOWN THE HOLE DRILL 4 Sheets-Sheet l June 9, 1964 E. B. LEAR ETAL 3,136,375

DOWN THE HOLE DRILL ATTORNEY June 9, 1964 E. B. LEAR ETAL DowN THE HOLE DRILL 4 Sheets-Sheet 3 Filed June 7, 1961 1. l .Il

June 9, 1964 E. B. LEAR ETAL 3,136,375

Down THE HOLE DRILL Filed June 7, 1961 4 Sheets-Sheet 4 INVENTOR ATTORNEY United States Patent O 3,136,375 DOWN THE HOLE DRILL Earl B. Lear and Alfred F. Hallett, Jr., Utica, N.Y., assignors to Chicago Pneumatic Tool Company, New York, NX., a corporation of New Jersey Filed June 7, 1961, Ser. No. 115,493 7 Claims. (Cl. 173-16) This invention relates to a pneumatically powered percussive rock drill of down-the-hole type that is adapted to be coupled -to a rotatable stringer and lowered into a drilled hole to effect drilling operations at the bottom of the hole.

In conventional drills of this type a piston is pneumatically reciprocated to pound a drill bit. This has created a problem as 4to how to effectively relieve the piston cylinder of air spent in driving the piston so as not to impede its operating efficiency.

An object of this invention is to provide practical and efficient means for relieving the piston cylinder of air spent in reciprocating the piston, and also for utilizing such relieved air in clearing the bottom of the hole below the drill bit of cuttings so as to enable a continuous benecial operation of the drill bit.

It is essential that the vents used in relieving the piston of spent air be maintained at all times in open condition to avoid the loss of time, wasted labor, and expense that would result if such vents were repeatedly clogged and required repeated drawing of the drill from the hole to clear them.

A further object of this invention is to provide practical and eflicient means for maintaining these relief vents clear Aand fully open at all times during the operation of the drill.

Another object of this invention is to provide means in a drill of this type, which enables a direct ow of live pressure fluid through various passages in the air exhaust system of the drill to clear and maintain such passages clear of dirt and especially foreign particles obstinately lodged in the passages.

In drills of this type it is expedient to stop the reciprocating action of the piston when the drill is lifted clear of the bottom of the hole, otherwise continued pounding actions ofthe reciprocating piston might cause serious damage to the component parts of the drill. Anything `less than complete stopping of the action of the piston vmay tend to cause damage.

`clear of the bottom of the hole; and which, while the piston is so stopped, will cause live air to be diverted through the drill for hole cleaning purposes.

A further object of this invention is to provide in a drill of this type an improved piston structure having an advantageous stem portion at one end cooperating with a recessed member to function as an air cut-off valve.

Another object of this invention is to provide a down- `the-hole type drill which is eicient in its operation, prac- Patented June 9, 1964 ice tical in its construction, economical in its use, and which, because of these various features, represents a decided advance over conventional drills.

The invention further lies in the particular construction and novel arrangement of its various components, and also in their cooperative association with one another to effect the benefits intended herein.

The foregoing and other objects and advantages of this invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein an embodirnent-of the invention is illustrated. It is to be expressly understood, however, that the drawings are for purposes of illustration and description, and they are not to be construed as defining the limits of the invention.

In the accompanying drawings:

FIGS. lA and 1B are continuations of one another, and together illustrate a longitudinal sectional view of a drill unit embodying the invention, FIG. 1A representing the upper half of the unit, and FIG. 1B representing the lower half; i

FIG. 2 is a rear end View of the portion of the drill unit shown in FIG. 1A; the stringer element, shown in broken line in FIG. lA, being omitted;

FIG. 3 is a section taken on line 3-3 of FIG.1A;

FIG. 4 is a section taken on line 4--4 of FIG. 1A;

FIG. 5 is a front or bottom end view of that portion of the drill unit shown in FIG. 1B;

FIG. 6 is a section taken on line 6-6 of FIG. 1B;

FIG. 7 is a section taken on line 7-7 of FIG. 1B;

FIG. 8 is a fragmentary sectional view showing the striking bar having the normal operating position shown in FIG; 1B wherein the drill bit is pressed by the load of the casing upon the bottom of the hole, but showing the air supply control valve to have shifted from the position shown in FIG. 1A and showing the piston in its returned position;

FIGS. 9A and 9B`are continuations of one another showing the drill unit elevated clear of the bottom of the bore hole;

FIG. 9A is a fragmentary section of the drill unit showing the piston and striking bar in their lower most positions relative to the drill casing;

. FIG. 9A shows the terminal portion of the bit elevated clear of the bottom of the bore hole; and

FIG. l0 is a View similar to FIG. 9A but showing a modification in the means for diverting pressure iiow through the piston and striking bar to vents in the bit.

In the drawings is disclosed a down-the-hole drill unit v1l adapted to be axially aligned and coupled to a rotatable Stringer 12 for unitary rotation with the latter. As the Stringer and drill rotate and are fed downward in conventional manner, a drill bit 13 at the bottom end of the drill boresinto the earth to form a hole 14. The resultant hole is only slightly wider in diameter than the drill unit and Stringer. Pressurized air admitted in conventional manner to the Stringer ows into the drill unit. A flow control V-alve unit 15 within the drill directs this pressure air alternately to opposite ends of ,a hammer piston 16 so as to reciprocate the latter. On its downward stroke the piston pounds an anvil or striking bar 17. This pounding causes the bit 13, which is mounted to the bottom of the striking bar, to bite into the rock strata 1S at the bottom of the hole. Air spent in reciprocating the piston is' exhausted from the drill to the bottom of the hole through vents 19 in the bit. The exhausted air acts to clear away chips from the bottom of the hole, and thus avoids recutting of the chips by the bit. Ecient operation of the drill requires that the vents 19 in the bit be kept open to enable the escape or exhaust of pressure air spent in operating the piston. The exhausting air also serves to drive and clear dirt out of the vents. The structure of the drill is further designed so that at various times live air at intake pressure may be effective diverted from reciprocating the piston and caused, while the piston is stopped, to liow through suitable passages leading to the vents 19 to Vclean the latter.

The drill includes in its assembly an outer cylindrical casing 21, the interior of which is also cylindrical and open at its ends. Fitted in the open ends of the casing are caps 22 and 23. The front or lower cap 22 has an external nut shaped head 24 to which a Wrench may be applied; and itV also has a thickened body section 2.5 threaded tightly into the front end of the casing. An `O-ring 26 seals the casing iiuid tight around the body of the cap. The rear or upper cap 23 has an external nut shaped head 27 to which a wrench may be applied; and also has a body section 28 threadedly drawn into the rear end of the casing. A shoulder 29' of cap 23 limits against therrear end of the casing. An O-ring 31 seals the rear end of the casing fluid tight against leakage around the body of cap 23. The drill unit 11 is adapted to be removably coupled in axial alignment to the drill Stringer Y12?. by means of a threaded nipple 32 of the rear cap 23. This nipple is threadably engageable in a complementary end 33 of the Stringer, as in FIG. 1A.

The rear cap 23 has an axial main intake passage 34 for admitting pressure air from the Stringer to the interior of the casing 21. Pressure air flowing through passage 34 is cleaned of dirt by means of an air lter or strainer 35 before it is permitted to pass into the valve unit 15. Cleaning of the air is desired to avoid blocking of the valve unit and scoring or other damage which might otherwise occur if foreign particles were admitted to the valve unit and other parts of the drill. The strainer i 35 is of disc .form and is retained in a deep annular recess 36 formed in the bottom end of the rear cap. The strainer is fixed across the top of a depending annular skirt 37. The strainer is restrained in the recess against endwise movement by means of a shoulder 38 of the recess and by the upper annular surface of a valve guide member 41 of the valve unit 15. The recess 36 in cap 23 extends above the strainer to provide an enlarged space 42 between the strainer and the terminal end of the intake passager 34. This enlarged space 42 permits dirt particles screened from the intake airto be freely blown about by the incoming air and thus prevented from settling and clogging the strainer during a normal period of operation of the drill.

After leaving the strainer, the cleaned pressure air enters the ow control valve unit 15. The latter includes the valve guide member 41. This member has an axially depending cylindrical shaft 43 upon which is sleeved a slide valve 44. The latter is shiftable up and down the shaft 43 in a valve chamber 45. The latter chamk ber is defined by a surrounding valve chest comprising front and rear annular sections 47 and 48. The rearchest section 47 limits at one end against the underface of an annular radial flange 49 of the valve guide member. The opposite marginal face portion of flange 49 limitsagainst the end wall 51 of the rear cap.

The front chest section 48 limits at one end against the forward end of the rear chest section 47, and limits at its opposite end against the rear annular end wall 52 of an open ended liner 53. The latter lines the interior wall ofthe casing 21. A rear end portion of they liner sleeves over a reduced annular front extension 54 of the front chest section 48. The lower end wall'55 of the liner limits against the inner end wall 56 of the front cap 22. Proper adjustment of lthe front cap retains the liner 53 and valve'unit 15 in a tightly assembled condition. Suitable aligning pin elements, one being shown at 57 (FIG. 1A), serve to prevent turning of the liner and the components of the valve unit relative to one another and relative to the casing. A piston chamber or cylinder 53 is defined by the interior of the liner 53.

Pressure air leaving the strainer 35 enters through a plurality of passages 59 in the valve unit to the valve chamber 45. Valve 44 has a normal or down position, as in FIG. 1A, wherein an annular shoulder 61 thereof seals over a complementary seat or" the valve chest and blocks flow of pressure air from the valve chamber through a plurality of longitudinal ports 63 to the rear end of the piston cylinder. rl`he valve has a shifted reverse or up position, asin FIG. 8, wherein a rear shoulder 64 of the valve seals over a complementary seat of the valve chest and blocks olf liow of pressure air from the valve chamber through radial ports 66 to a plurality of longitudinal arcuate channels 67 in the surface of the liner. The latter channels lead to radial ports 68 communicating with the lower end of the piston cylinder (FIG. 1B). Pressure air admitted alternately to opposite ends of the piston cylinder 5S causes the piston 16 to reciprocate.A Air trapped at veither end of the piston cylinder is returned over correspondingend passages to the valve unit under pressure of the reciprocating piston, vand serves to automatically shift the valve 44 from one position to the other.

The reciprocating piston 16 operates to pound the anvil member or striking bar 17. The latter has a cylindrical anvil head 69 which is axially slidable in the lower end of the piston cylinder 58, the outer peripheral surface 69 being in sliding contact with the wall of the cylinder. Axially depending from the anvil head 69 is an elongated shank portion '71 of reduced diameter having elongated longitudinal splines 72 which slidably interiit with complementary but relatively shorter splines 73 formed in an, axial bore of the front cap 22, whereby rotation of the casing is imparted to the striking bar. A screw 75 defined by an extended terminal portion of the shank 71of the striking bar projects through the bottom end of 'the front cap. This screw 75 is threaded tightly into a complementary recess 76 of the bit 13. A shoulder 77 defined by the rear end of the bit is cooperable, as appears Vin FIG. 1B, with the bottom end of the front cap to limit theextent to which the striking bar may be elevated in the piston cylinder 58. ln this elevated position of the striking bar, an upper annular shoulder 78 of its head is disposed below the radial lower inlet ports 68 of the piston cylinder; and a reduced upper end 79 of the head is disposed directly opposite this port to provide an annular space S1. This space provides a desired area or chamber for the admission of pressure air from ports 68 -to the lower end of the piston cylinder to effect a return action of the piston 16. An annular shoulder 82 at the bottom of the head of the striking bar cooperates with the upper end wall 56 of the front cap to limit the extent of downward movement of the striking bar and ofthe bit to the position shown in FIG. 9A.

The striking bar assumes the low position shown in FIG.'9A relative to the casing 21 during the time the drill unit is being raised or lowered into a hole.

The piston 16, which operates to pound the striking bar,has a level end face 83 adapted to pound an opposed level anvil face 84 of the striking bar. In either the impacting or normal operating position shown in FIGS. lA and 1B; or in the low position of the striking bar shown in FIGSJQA and 9B, the end face 83 of the piston abuts theanvil face-84 of the striking bar, and an axial stem 85 of the striking barV is slidably received in an enlargement 86 of the lower end of a bore 87 extending axially through the piston. The piston bore 87 is aligned with a bore S8 extending axially through the striking bar. The latter bore communicates in the bit with the vents v of greater length than the' stem 85 of the striking bar extends from the upper end of the piston. This stern is adapted on the return stroke of the piston to be slidably received as in FIG. 8 in a complementary recess 91 formed in the valve guide member 41. It is to be noted in FIG. 1A and 1B that the'piston stem 89 is clear of the recess 91 and of the valve guide 41 when the piston abuts the anvil face 84 of the striking bar. This position of the piston enables air spent in driving the piston downward to exhaust through the bores 87 and 88, and the vents 19 of the bit. It is to be noted in FIG. 8 that the piston is in its returned position and it is clear of the stem 85 of the striking bar, thus enabling air that has been spent in returning the piston to exhaust through the bore 88 of the striking bar, and the vents 19 of the bit.

Inv summarizing the operation of the drill unit, reference is directed to FIGS. 1A and 1B wherein the bit 13 is pressed under the load of the drill `casing 21 and stringer 12 against the rock bottom 18 of the bore hole 14. This is the normal operating condition of the drill unit. In this condition of the drill, pressure air from the valve chamber 45 passes around the valve shoulder 64 to the radial ports 66the longitudinal passages 67, and then to the lower inlet ports 68 to the annular space 81. Pressure air entering space 81 acts against the marginal annular area of the end face 83 of the piston, and drives the piston upwardly to its return position. As the piston moves upwardly and clears the stem 85 of the striking bar, spent air at the lower end of the piston cylinder exhausts through the bore 8S of the striking bar and from the vents 19 of the bit. As the piston is returned, residual air in the cylinder above the piston is cut oi from reaching the exhaust bore 87 of the piston after the piston stem 89 slides into the recess 91 of the valve guide. This trapped residual air is forced through the front ports 63 of the valve unit against the valve shoulder 61 to shift the valve 44 to its upward position, so as to communicate the Valve chamber 45 with the rear end of the piston cylinder. Whereupon, pressure air from the valve chamber 45 next flows through the ports 63 of the valve unit to the piston cylinder 58 to act upon the rear end of the piston, causing the piston to move down and pound against the anvil face 84 of the striking bar and thereby cause the bit 13 to bite into the rock 18 below. The front cap 22 presses slidably relative to the striking bar downwardly at all times on the shoulder 77 of the bit, so that as the latter bites into the rock 18 the casing moves with the bit as a unit. On the impact stroke of the piston, residual air trapped by the piston is driven back through the inlet ports 68 and associated passages 66 to the valve unit against the valve shoulder 64 to reshift the valve down to its forward position. Air at the upper end of the piston cylinder that was spent in driving the piston on its impact stroke exhausts'through the bores 87 and 88, and the Vents 19 of the bit to the bottom of the hole 14. As the spent air exhausts through the slanted vents 19 ofthe bit, it not only clears away the chips at the bottom of the hole but also'clears the vents 19 of any dirt that may have entered the latter.

'When the drill unit is being elevated or lowered by the stringer in the bore hole 14, the bit 13 will not be supported by thebottom 18 of the hole, and because of this lack of support the striking bar and the piston will slide downward relative to the casing 21 to their lowermost positions as in FIGS. 9A and 9B wherein the shoulder 82 of the striking bar will abut the shoulder 56 of the front cap, and wherein the piston will abut the top end 84 of the striking bar. In this low position of the piston the body of the latter covers over the inlet ports 68 and is clear of an intermediate inlet port 94. The latter port is located about midway of the piston chamber 58 and is normally sealed over by the body of the piston during normal operation of the latter. n While the piston and striking bar are in the low position of FIGS. 9A and 9B, pressure air ows through the radial ports 66 of the control valve to the longitudinal passages 67 and through the port 94 to the upper end of the piston cylinder. From the latter it exhausts at main line pressure through the bores 87 and 88 and through the ventsy 19 to the hole 14, thereby maintaining the vents clear of dirt and also feeding pressure air to the bottom of the hole.

It is essential that the reciprocating and pounding action of the piston be completely stopped during the time that the drill unit is clear of the bottom 18 of thev hole 14. The pounding action of the reciprocating piston is nor- ,mally absorbed largely by the rock bottom 18 of the bore hole 14, but when the drill unit is oif the bottom of the hole this pounding force will be, unless the piston is cornv pletely at rest or stopped, absorbed by the components of the drill unit. When the latter occurs, serious damage may be caused to the components of the unit.

After the piston has limited in itslow position, as in FIGS. 9A and 9B, it would be expected that the piston will be stopped from reciprocating inasmuch as flow through the ports 68 is blocked by the covering body of the piston and flow through port 94 is exhausted through the bores 87 and 88, and vents 19. However, this is not the case, since there is slight leakage of pressure air from the passager 67 through ports 68 and around the body `of the piston to the annular space 81 at the underside of the piston. This leak-by air, unless relieved, will develop and cause the piston to creep upwardly to effect a shift movement ofthe control Valve and a consequent flow. of pressure air through the valve ports 63 to cause an undesirable 'forceful pounding action of the piston against the striking bar. v Y A feature of the present invention is the provision of means whereby this leak-by air is quickly and fully vented to atmosphere, so that when the piston has moved to its lowermost position (FIGS. 9A, 9B) it will remain cornpletely stopped in such position until the drill unit is again limited against the bottom of the hole and the striking bar has again moved upwardly relative to the casing to its normal position shown in FIG. 1B.

To the end of preventing leak-by air from developing 'in the space 81 below the piston, the piston cylinder liner 53 is internally enlarged or counterbored at its lower end to provide an annular recess 95. This recess has a vertical height which is greater than the longitudinal dimension of the outer surface 69a of the anvil head of the striking bar between the shoulders 78 and 82. Accordingly, when the striking bar is moved to its lowermost position, the annular outer surface 69a thereof will be clear of the wall of the liner 53 including that of the recess 95, and the latter recess will then be in direct communication with the space 81. Leak-by air under this arrangement will flow from the space 81 to the recess 95. The recess connects by means of a longitudinal bore 96 and a short radial bore 96a in the front cap with the interior of the latter just above the upper ends of the splines 73. The flutes or spline grooves 72a of the striking bar are relatively longer and radially deeper than the splines 73 of the front cap, so that in the lowermost position of the striking bar the upper end of a flute of the striking bar is in communication with the radial port 96a and longitudinal clearances are provided at 97 between the bottoms of the iiutes of the striking bar and the crests of the splines 72. Accordingly, leak-by air is able to freely pass from the` space 81 to the recess 95, radial port 96a and the flute clearance 97 to atmosphere. To avoid any side play of the striking bar in its low position (FIG. 9A) relative to the liner 53 and the front cap 22, the shank of the striking bar has a smooth peripheral surface portion 71a between the upper ends of its splines and the aisaars shoulder 82, which has a slide iit in a complementary internal portion of the front cap Z2.

In a modified construction of the drill unit, as indicated in the fragmentary drawing of FIG. l wherein the piston is in its low position, the port 94 shown in FIGS. 1A and 9A is omitted. Instead, a radially directed or transverse port 98 in the lower end of the piston is used. This port connects the axial bore S7 with a peripheral groove 99'; and the latter is adapted to register with ports 68 when the piston is in its lowermost condition, whereby pressure air from passage 67 passes through ports 68, piston port 98, and through the axial bores 87 and SS to the vents 19.

A blow plug 100 (FIG. 8) is removably threaded in an axial hole of a crowned end 101 of the valve guide member 4l. This plug is of advantage in that it'provides a hole 162 through which a restricted flow of live air is continuously fed from the main inlet passage 34 directly to the recess 91 of the valve guide member, whereby a restricted flow of pressure air continually ilows through the bores 87 and 88, and from the vents I9 to prevent entry of dirt into the latter. Flow through the plug lili) may be increased or decreased as desired by replacing the plugv with a diierent size hole 192.

y A further feature of the drill unit is provided by the distinctive construction of the piston. It is noted that the piston includes a stem portion 89. The latter is preferably a separate element which is press fitted fast into a counterbore 103 of the body of the piston. This construction is of advantage in that the stern may be readily replaced in the event it should be snapped 01T, as might occur if the piston while in an unassembled condition were accidentally dropped.l On the other hand, were the stem S9 formed as one piece with the body of the piston, not only would this integral structure be expensive to make, but if the stem were to be snapped oi the entire piston might have to be discarded.

What is claimed is:

l. A rock drill of a type adapted to be coupled to a rotatable Stringer and lowered down into a hole of slightly larger diameter to effect drilling at the bottom of the hole, comprising a casing having a piston cylinder therein, a striking bar in the lower end of the cylinder, and a piston pneumatically .reciprocable in the cylinder above the striking bar so las to directly pound the latter on its down stroke, the striking bar including an anvil head within the cylinder subject to being pounded by the piston and having a bit section externally of the casing, a iirst air passage opening into the cylinder below the piston, a second separate air passage opening into the cylinder above the piston, vthe casing having a head end provided with a main operating air supply passage, operating air control means having a slide valve shiftable in response to back pressures of air created thereon as the piston reciprocates sor-as to communicate the said iirst and second passages alternatelyl with the said supply passage, and venting means allowing substantial escape of spent air alternately from opposite ends of the .cylinder as the piston reciprocates, wherein the venting means includes axial bores through the piston and striking bar in direct communication with one another, the bit has vents through its bottom in communication with the bore of the striking bar, and means providing a restricted passage coaxial with the piston cylinder and with said bores continuously communicating the main operating air supply passage with the piston cylinder and with said bores, whereby dirt is prevented from accumulating in the latter for admitting a continuous restricted iiow of pressure air axially tothe upper end of the piston cylinder directly to the bore of the piston for flow through the striking bar and the vents of the bit.

2. A drill of the character described having a piston cylinder therein, a piston pneumatically reciprocable up and down within the cylinder over a normal range during normal operation of the drill, a drill element having a bi-t externally of the. casing and having an anvil head slidable within the cylinder, the anvilV head being actuable by the piston to cause the bit to bite into the earth, a rst passage opening into the lower portion of the cylinder for supplying'pressure driving i'luid to the under end of the piston to pneumatically return the latter on a return stroke, a venting passage opening into the piston cylinder at a certairl distance below the rst passage, the drill element having a normal operating position wherein the anvil head thereof cuts off communication of the irst passage with the vent passage, the piston having a low position beyond its normal range wherein it rcovers over the first passage, the sliding fit of the piston in its cylinder being such relative to the latter that there is some leak-by of air from the first passage around the piston to the under end of the latter when the piston is in its low position, and the drill element having a low position when the piston is in its low position wherein the anvil head is clear of the vent passage so that any pressure air that might leak from the lrst passage around the piston to the under end of thelatter is vented over the vent passage and thereby prevented from effecting any return movement of the piston lduring the time the piston is in said low position beyond its normal range.

3. In a rock drill of thecharacter described, a casing, a piston cylinder in the latter, a hammer piston reciprocable in the cylinder, a first passage opening into the lower end of the cylinder, a second passage opening into the upper end, a pressure air-supply control slide Valve arranged at the top end of the cylinder automatically shiftable relative to said first and second passages in response to pneumatic back pressures created alternately over said passages as lthe piston reciprocates to charge opposite ends of the cylinder with pressure air alternately through said passages, a drillfelement having a bit externally of the casing andan anvil head slidable in the lower end of the cylinder, an axial passage through the piston, an axial passage through the drill element terminating in a plurality of vents in the bit, the anvil head having a normal operating position wherein it is clear of the irst opening, the piston having a normal impacting position, against the anvil head wherein it is above the first opening, an

.axially recessed member fitted in the upper end of the cylinder, open ended stem means on the upper end of the piston cooperable with the recess ofthe recessed member for cutting oi the axial passageV of the piston horn the upper end of the cylinder on a return stroke of the piston, a counterbore in the bottom end of the piston, open ended stern means on the upper end of the anvil head cooperable with the counterbore for cutting off communication of the axial passage of the drill element from the cylinder on theI impacting stroke of the piston, a main intake pressure air supply passage connected with the control valve, and a restricted opening in the main intake passage communicating axially with the recess of the recessed member providing at all times a restricted axial flow of pressure air directly to the upper end of the axial passage of the piston.

4. In a rock drill according to claim 3, wherein an air lter screen is interposed between the intake opening and both the restricted opening and the control valve.

5. In a rock drill accordingto claim 4, wherein an enlarged recess is provided between the intake opening and the screen to allow screen-out dirt to be freely blown about by the intake air above the screen so as to prevent such dirt from settling and clogging the screen.

6. In a rock drill according tto claim 3, wherein a vent passage is provided at the lower end of the cylinder a certain distance below the iirstl opening, and the anvil head has an enlarged peripheral surface in contact with the wall of the cylinder between the vent passage and the rst opening whereby the latter is normally cut olf from the former.

7. In Va rock drill according to claim 6, wherein the anvil head has a low position 'other than its normal posisage opening.

References Cited in the le of this patent UNITED STATES PATENTS Mercer Dec. 2, 1924 Lear Oct. 24, 1939 10 Topanelian Dec. 25, 1951 10 Y Bassinger Feb. 12, 1952 Bassinger July 14, 1953 Bassinger Jan. 5, 1954 Mori July 30, 1957 Fish Sept. 30, 1958 Bassinger et al Nov. 11, 1958 Morrison May 12, 1959 Feucht Aug. 2, 1960 Bassinger Apr. 11, 1961 Vincent et al Dec. 12, 1961 Stall et al. Aug. 27, 1963 

1. A ROCK DRILL OF A TYPE ADAPTED TO BE COUPLED TO A ROTABLE STRINGER AND LOWERED DOWN INTO A HOLE OF SLIGHTLY LARGER DIAMETER TO EFFECT DRILLING AT THE BOTTOM OF THE HOLE, COMPRISING A CASING HAVING A PISTON CYLINDER THEREIN, A STRIKING BAR IN THE LOWER END OF THE CYLINDER, AND A PISTON PNEUMATICALLY RECIPROCABLE IN THE CYLINDER ABOVE THE STRIKING BAR SO AS TO DIRECTLY POUND THE LATTER ON ITS DOWN STROKE, THE STRIKING BAR INCLUDING AN ANVIL HEAD WITHIN THE CYLINDER SUBJECT TO BEING POUNDED BY THE PISTON AND HAVING A BIT SECTION EXTERNALLY OF THE CASING, A FIRST AIR PASSAGE OPENING INTO THE CYLINDER BELOW THE PISTON, A SECOND SEPARATE AIR PASSAGE OPENING INTO THE CYLINDER ABOVE THE PISTON, THE CASING HAVING A HEAD END PROVIDED WITH A MAIN OPERATING AIR SUPPLY PASSAGE, OPERATING AIR CONTROL MEANS HAVING A SLIDE VALVE SHIFTABLE IN RESPONSE TO BACK PRESSURES OF AIR CREATED THEREON AS THE PISTON RECIPROCATES SO AS TO COMMUNICATE THE SAID FIRST AND SECOND PASSAGE ALTERNATELY WITH THE SAID SUPPLY PASSAGE, AND VENTING MEANS ALLOWING SUBSTANTIAL ESCAPE OF SPENT AIR ALTERNATELY FROM OPPOSITE ENDS OF THE CYLINDER AS THE PISTON RECIPROCATES, WHEREIN THE VENTING MEANS INCLUDES AXIAL BORES THROUGH THE PISTON AND STRIKING BAR IN DIRECT COMMUNICATION WITH ONE ANOTHER, THE BIT HAS VENTS THROUGH ITS BOTTOM IN COMMUNICATION WITH THE BORE OF THE STRIKING BAR, AND MEANS PROVIDING A RESTRICTED PASSAGE COAXIAL WITH THE PISTON CYLINDER AND WITH SAID BORES CONTINUOUSLY COMMUNICATING THE MAIN OPERATING AIR SUPPLY PASSAGE WITH THE PISTON CYLINDER AND WITH SAID BORES, WHEREBY DIRT IS PREVENTED FROM ACCUMULATING IN THE LATTER FOR ADMITTING A CONTINUOUS RESTRICTED FLOW OF PRESSURE AIR AXIALLY TO THE UPPER END OF THE PISTON CYLINDER DIRECTLY TO THE BORE OF THE PISTON FOR FLOW THROUGH THE STRIKING BAR AND THE VENTS OF THE BIT.
 2. A DRILL OF THE CHARACTER DESCRIBED HAVING A PISTON CYLINDER THEREIN, A PISTON PNEUMATICALLY RECIPROCABLE UP AND DOWN WITHIN THE CYLINDER OVER A NORMAL RANGE DURING NORMAL OPERATION OF THE DRILL, A DRILL ELEMENT HAVING A BIT EXTERNALLY OF THE CASING AND HAVING AN ANVIL HEAD SLIDABLE WITHIN THE CYLINDER, THE ANVIL HEAD BEING ACTUABLE BY THE PISTON TO CAUSE THE BIT TO BITE INTO THE EARTH, A FIRST PASSAGE OPENING INTO THE LOWER PORTION OF THE CYLINDER FOR SUPPLYING PRESSURE DRIVING FLUID TO THE UNDER END OF THE PISTON TO PNEUMATICALLY RETURN THE LATTER ON A RETURN STROKE, A VENTING PASSAGE OPENING INTO THE PISTON CYLINDER AT A CERTAIN DISTANCE BELOW THE FIRST PASSAGE, THE DRILL ELEMENT HAVING A NORMAL OPERATING POSITION WHEREIN THE ANVIL HEAD THEREOF CUTS OFF COMMUNICATION OF THE FIRST PASSAGE WITH THE VENT PASSAGE, THE PISTON HAVING A LOW POSITION BEYOND ITS NORMAL RANGE WHEREIN IT COVERS OVER THE FIRST PASSAGE, THE SLIDING FIT OF THE PISTON IN ITS CYLINDER BEING SUCH RELATIVE TO THE LATTER THAT THERE IS SOME LEAK-BY OF AIR FROM THE FIRST PASSAGE AROUND THE PISTON TO THE UNDER END OF THE LATTER WHEN THE PISTON IS IN ITS LOW POSITION, AND THE DRILL ELEMENT HAVING A LOW POSITION WHEN THE PISTON IS IN ITS LOW POSITION WHEREIN THE ANVIL HEAD IS CLEAR OF THE VENT PASSAGE SO THAT ANY PRESSURE AIR THAT MIGHT LEAK FROM THE FIRST PASSAGE AROUND THE PISTON TO THE UNDER END OF THE LATTER IS VENTED OVER THE VENT PASSAGE AND THEREBY PREVENTED FROM EFFECTING ANY RETURN MOVEMENT OF THE PISTON DURING THE TIME THE PISTON IS IN SAID LOW POSITION BEYOND ITS NORMAL RANGE. 